Help finding replacement / duplicating bias transformer

[bjoneson]

Background... working on getting a Scully 284B-8, 1" 8 track machine up and running. The electronics consist of 4, 4U modules, each of which contains 2 channels worth of electronics. Additionally... the first module also contains the master bias oscillator, and each of the 3 other modules contain a bias amplifier.

My conundrum is I have only 2 of the bias amplifier cards, and need a 3rd. They're virtually unobtanium, so I've resigned to etching / reproducing one. The circuit and layout is trivial, and I can source all the components I need, except for the transformer.

I've attached a schematic of the bias amplifier circuit, as well as a picture of the transformer from one of the cards I have, I'm attempting to duplicate.

In the schematic, it appears to be a simple step down transformer (shows 40V P-P on the primary, 5V P-P on secondary). The 160KHz bias signal is fed from the master bias oscillator to this amplifier card via P103(3) on the schematic.

I assume based on the voltages listed, it's an 8:1 turns ratio?

I'm thinking of potentially just winding my own, if I can't find a suitable drop in using the EPCOS stuff from TDK, but I have absolutely no idea where to start in terms of specs, what size core, what gauge magnet wire, etc...

Any help is greatly appreciated!

Many thanks,

Bob

 

[bjoneson]

Pic of xfrmr

 

[PRR]

> a simple step down transformer

Well, but it has a significant capacitor across a winding.

Smells like a tuned circuit.

Then again, if pin 3 drive is a low-Z source, 1K resistance feeding 0.001uFd resonant at 160KHz suggests a Q very near unity. Hardly a sharp filter.

I'm wondering why it even has a transformer. Ground isolation? No, because pins E1 and E3 seem to go to the same "ground".

And no precision is needed on amplitude because R6 is a trim.

I'm wondering why not a 8:1 divider (say 33K in series with R6 5K) from R16. 500pFd in place of C8 and a 180pFd across R6 gives some low-pass action to reduce the even-order harmonics of the 160KHz signal.

(They may not have been too obsessed about even harmonics, because that power amp is crude and will throw some 2nd Harmonic, perhaps 1%?)